1. Field of the Invention
The present invention relates to a measurement apparatus, and more particularly to a measurement apparatus which measures a position of an object to be measured with high accuracy based on a signal whose phase changes in accordance with the change of the position of the object to be measured.
2. Description of the Related Art
Conventionally, as a detecting unit which detects a position or an angle of an object to be measured, a detecting unit such as an encoder, a resolver, and a laser interferometer is industrially widely used. These detecting units output a plurality of signals whose phases change in accordance with the change of the position or the angle of the object to be measured.
Some detecting units output two-phase signals corresponding to a sine and a cosine, and other detecting units output their inverse signals. Furthermore, there are detecting units which output three-phase signals whose phases are 120° different from one another.
As a means for obtaining fine position information or angle information using these signals, a method of converting each signal that is an analog signal into a digital signal and performing an arc tangent calculation or the corresponding arithmetic processing by a digital arithmetic unit is widely used.
In the present age, an analog to digital converter which has a resolution of 14 to 16 bits can be easily obtained. The phase information which divides one cycle of a signal into equal to or more than several ten thousands can be obtained by the digital calculation if these analog to digital converters are used.
However, the signal outputted from the detecting unit is an analog signal. Therefore, an error (distortion) relative to an ideal state occurs in a zero point, an amplitude, and a phase difference between the signals caused by the temperature change or the change with the passage of time. Generally, in the detecting unit which divides one cycle of the signal into several hundreds, these distortions can not be ignored. The detecting unit operates by the power or the light beam which is provided, and also the intensity change of the power or the light beam by a noise can not be ignored.
The technology which removes the distortion component contained in a signal has been known from a long time ago. For example, U.S. Pat. No. 4,458,322 proposes a technology which removes an offset (a zero point) error and an amplitude error by processing a sine and a cosine signals outputted from an optical encoder. Japanese Patents No. 2,790,863, No. 2,893,340, No. 2,839,340, and No. 2,839,341, and U.S. Pat. No. 5,581,488 propose a technology which removes a phase error and an error caused by a high-harmonic distortion other than the errors of the zero point and the amplitude.
However, in all of the above references, an error coefficient which causes a distortion is calculated using a signal outputted from the detecting unit as it is. Therefore, a variety of errors contained in the signal superimposedly act on the signal and complexly deforms the output signal of the detecting unit. As a result, it was difficult to estimate the contained amount of the distortion component with high accuracy, and the detected phase contained a great error.
With respect to the signal outputted from the detecting unit, the electrical noise superimposed on a power supply line, the intensity change of the light source specifically in an optical detecting unit, and the like, other than the distortion component, can not be ignored. The electrical noise and the intensity change of the light source and the like was also a factor of the error contained in the measured phase.